'Unwell' Stephen Hawking misses 70th birthday event

British scientist Stephen Hawking was forced to miss a scientific debate to mark his 70th birthday Sunday due to ill health but sent an upbeat message saying he was living at a "glorious time".

As scientists and media gathered at Cambridge University for a symposium to mark Hawking's birthday, vice-chancellor Professor Sir Leszek Borysiewic announced: "Stephen has been unwell and was only discharged from hospital on Friday."

But in a pre-recorded message, the physicist urged participants to focus on his glittering career and the future of science rather than his struggles against illness.

"It has been a glorious time to be alive and doing research in theoretical physics," he said.

"We must also continue to go into space for the future of humanity," he argued. "I don't think we will survive another thousand years without escaping beyond our fragile planet."

Hawking, who was diagnosed with motor neurone disease aged 21, followed the special symposium on "the state of the universe" via webcast, the vice-chancellor said.

Despite spending most of his life in a wheelchair and being able to speak only through a computer, the theoretical physicist's quest for the secrets of the universe has made him arguably the most famous scientist in the world.

When Hawking was diagnosed with the debilitating condition he was given only a few years to live, but has defied medical opinion by reaching his eighth decade.

Hawking touched upon this time during Sunday's speech, saying that "every new day became a bonus" after accepting the extent of his illness.

The physicist also took a light-hearted look back at his schooldays, when he admitted he "was never more than about halfway up the class."

"When I was 12, one of my friends bet another friend a bag of sweets that I would never come to anything," he recalled. "I don't know if this bet was ever settled and, if so, which way it was decided."

Much of Hawking's work has centred on bringing together relativity (the nature of space and time) and quantum theory (how the smallest particles in the universe behave) to explain the creation of the universe and how it is governed.

His fame moved beyond academia in 1988 with the publication of his book "A Brief History of Time", which explained the nature of the universe to non-scientists, and sold millions of copies worldwide.

Hawking's stardom was later cemented in cameos in "Star Trek" and "The Simpsons", where he tells the rotund Homer Simpson that he likes his theory of a "doughnut-shaped universe", and may have to steal it.

As Hawking's age advances, he could be at risk of losing his famous computerised voice due to the gradual loss of muscle control, his personal assistant said ahead of his birthday.

"His speech has got slower and slower and on a bad day he can only manage about one word a minute," Judith Croasdell said in the Daily Telegraph.

Martin Rees, Britain's Astronomer Royal and a former president of the Royal Society, who first met Hawking when they were both research students, marvelled at his longevity.

He admitted that when they first met, "it was thought he might not live long enough to finish his PhD degree".

"His fame should not overshadow his scientific contributions because even though most scientists are not as famous as he is, he has undoubtedly done more than anyone else since Einstein to improve our knowledge of gravity."

Professor Kip Thorne from California Institute of Technology called Hawking a "superb human being and an inspiration".

"He's the most stubborn person I've ever met and that has to be part of his success," he continued.

"He finds some way to attack it and he doesn't say no with regard to his physical disability, he finds way to overcome it and become all the stronger for it."

Hawking has amyotrophic lateral sclerosis (ALS), a form of motor neurone disease that attacks the nerves controlling voluntary movement.

He concluded Sunday's message, which received a standing ovation, by imploring those present to "remember to look up at the stars and not down at your feet".

"Try to make sense of what you see and wonder about what makes the universe exist," he added. "Be curious. And however difficult life may seem, there is always something you can do and succeed at."